The present invention relates to a switched capacitor filter and, more particularly, to a switched capacitor filter for processing a plurality of signals on a time-division basis.
Filters which may be implemented by metal oxide semiconductor (MOS) integrated circuit technologies include a switched capacitor filter (SCF). An SCF comprises at least one operational amplifier (op amp), a plurality of capacitors, and a plurality of switching elements. Due to the SCF's frequency characteristic dependent upon a sampling frequency and a capacitance ratio, the SCF promotes easy production of precision filters without the need for adjustment. Such SCFs include a time-division multiplex switched capacitor filter as disclosed in the paper by Patrick W. Bosshart entitled "A Multiplex Switched Capacitor Filter Bank," IEEE Journal of Solid-State-Circuits, Vol. SC-15, No. 6, December 1980, pp. 939-945. The advantage attainable with the disclosed multiplex SCF is that since a single op amp is shared by a plurality of filters, it cuts down the chip area and power consumption when built in an integrated circuit. However, as will be described later in detail, such a conventional multiplex SCF develops different DC offset voltages in the different channels of incoming time-division multiplex signals. The DC offset voltages introduce errors in the event of rectification of filter outputs or detection of peaks. This requires the SCF to be furnished with offset adjusting circuits in one-to-one correspondence with the respective channels to remove the DC offset voltages, resulting in a complicated circuit arrangement.